1. Field
The invention relates to an austenitic manganese steel strip and to a process for the production of austenitic manganese steel strips. The invention further relates to a manganese sheet steel comprising a reshaped sheet steel portion, in particular a stretch-formed or deep-drawn sheet steel portion.
2. Description of the Related Art
Manganese austenites are lightweight structural steels which are particularly tough and, at the same time, can stretch. The reduction in weight made possible by the greater strength makes manganese austenites a material which has high potential within the automotive industry. This is because the fuel consumption can be reduced as a result of lighter bodies, a high level of elongation capability and stability being important for the production of body parts and for their behaviour in crash conditions.
Transformation Induced Plasticity (TRIP) steels are already known and are increasingly used within the automotive industry. High-alloy TRIP steels achieve high tensile strengths up to more than 1000 MPa and may have stretching abilities up to approximately 30%. Owing to these high mechanical properties, thinner sheet metals and thus a reduction in body weight can be achieved in automotive construction. TRIP steel consists of a plurality of phases of iron-carbon alloys, substantially formed of ferrite, bainite and carbon-rich residual austenite. The TRIP effect is based on the deformation-induced conversion of the residual austenite into martensite. This remodelling of the crystal structure causes a simultaneous increase in strength and formability during product production or during product use in the event of a crash. The TRIP effect can be selectively influenced by adding the alloy elements of aluminium and silicon.
With TRIP steel a specific amount of the austenite has already been converted during the deep-drawing of the body part into the high-strength martensitic phase (α-martensite), which can hardly be stretched. It is therefore possible that, in the case of TRIP steels, only a relatively low elongation reserve still remains for a crash situation.
The recently developed TWIP steels differ from TRIP steels in that they have a higher elongation at break (50% and above). The abbreviation TWIP stands for twinning induced plasticity, i.e. a plasticity which is induced by twinning. The specific stretching ability of TWIP steels can be produced by different mechanisms in the crystal structure. For example, the stretching ability may be promoted by lattice defects in the crystal structure, where the crystal structure may shear in a deformation-inducing manner, the shear mechanism taking place at a mirror plane and producing regularly mirrored crystal regions (‘twins’). It is possible to distinguish between different twinning types. It is further known that other effects, such as the occurrence of slip bands, may influence the mechanical properties. Owing to the high stretching ability, TWIP steels are excellently adapted for the production of sheet metals within the automotive industry, in particular for regions of the body which are relevant in the event of a crash. TWIP steels have an austenitic structure and are characterised by a high manganese content (normally above 25%) and relatively high alloy additions of aluminium and silicon.
A problem addressed by the invention may lie in the provision of a steel having improved mechanical properties. In particular, a good level of weldability of the steel and/or a good level of formability are to be obtainable. Furthermore, the invention aims to provide a process for producing a steel having improved mechanical properties, in particular high ductility in combination with high tensile strength, and in particular good weldability and good formability.